Unitless scales

[SeanDS]

Unitless scales

[KenKundert]

Thank you.

The way I would handle this situation would be to first apply units to the value, then do the conversion. So, something like:

UnitConversion('V/V', 'dB', from_dB, to_dB)                                      
gain = Quantity(10, 'V/V')
print(gain.scale('dB'))

or

UnitConversion(('V/V', 'A/A'), 'dB', from_dB, to_dB)

Vgain = Quantity(10, 'V/V', scale='dB')
Igain = Quantity('1M', 'A/A', scale='dB')                                            
print(Vgain, Igain)

[KenKundert]

Alternatively, you can avoid using UnitConversion like:

def to_dB(value, units=''):
    return 20*math.log10(value), 'dB'

gain = Quantity(10, scale=to_dB)
print(gain.scale('dB'))

(In answering this question I discovered that the scaling functions used by Quantity are different than those used by UnitConversion. That was a surprise and does not seem ideal, but I have not decided how to make them consistent. Thus, the to_dB function used in this answer is different from the one used in the previous answer.)

[SeanDS]

Your first approach of applying units to the quantity seems like the best approach for my use-case. Thanks!

BTW, does quantiphy detect scales from parsed strings? Ideally I'd like to be able to be able to parse strings with or without dB without first looking at the string to see if it has a scale, i.e. the following condition should be true:

Quantity("120 dB") == Quantity("1e6")

Is this possible or planned?

[KenKundert]

QuantiPhy will not scale unless you tell it to, but I think you can get what you want with something like this:

>>> UnitConversion('V/V', 'dB', from_dB, to_dB)

>>> for gain in '100 100V/V 40dB'.split():
...     lgain = Quantity(gain, 'V/V', scale='V/V')
...     print(f'{gain:>6} → {lgain}, {lgain:pdB}')
   100 → 100 V/V, 40 dB
100V/V → 100 V/V, 40 dB
  40dB → 100 V/V, 40 dB

In this example gain is a string that may or may not have units. If it does not have units, Quantity will use the second argument as the default units. Finally the third argument says to convert the given value to 'V/V'. The first two values will not be affected because they already are 'V/V', but the third value is 'dB', so Quantity will use from_dB to convert it to 'V/V'.